A network of the invention may include certain items that are common, as to their functions mentioned below, with a first known network. These items include:
a plurality of terminals respectively associated with as many user peripherals between which messages are to be transmitted;
two optical fibers associated with each of said terminals to guide optical waves, said fibers comprising an emission fiber for guiding emission waves emitted at an emission frequency from the terminal and a reception fiber for guiding waves received by the terminal; and
a star coupler for receiving the optical waves that reach it via said emission fibers and for transmitting each of said optical waves to all of the other terminals via said reception fibers.
The emission frequencies of the various terminals vary over time. However, to avoid cross-talk, they must remain different and separated from one another by spectrum distances of not less than a frequency increment. The frequency increment is selected to reserve a message channel of sufficient bandwidth for each call in the spectrum range of the network. Thus starting from a base frequency, these frequencies form a sequence referred to below as a "stack" and which extends up to a stack top. Said sequence corresponds to a sequence of message channels which include said frequencies and also to a sequence of terminals which use said frequencies. In each of said frequencies, an item may be adjacent to another item, i.e. it may precede it or it may follow it. The top of the stack is constituted by one of the emission frequencies, or when a call is made over a plurality of frequencies by a group of such adjacent frequencies, with said frequency(ies) being furthest from the base frequency.
The various terminals may be identical to one another. That is why to facilitate the description of the operation of such a network one of the terminals is sometimes considered more particularly, which terminal may be constituted by any of the terminals and is referred to as the "terminal under consideration".
The said first known network is described in a first prior document constituted by European patent document EP-A-0 381 102 (F.degree.16761). That document explains how the emission frequency of the terminal under consideration is established while is it preparing to enter into communication. The frequency is established by means of a "support". Such a "support" consists in the terminal under consideration establishing its emission frequency relative to a support frequency in such a manner that the spectrum distance between said emission frequency and said support frequency is equal to said frequency increment. If possible, the support is bilateral, i.e. it is taken from two support frequencies constituted by two emission frequencies that are separated by twice the frequency increment. Both of these frequencies then become adjacent to the emission frequency of the terminal under consideration and respectively constitute lower and upper support frequencies which precede and follow said emission frequency. In the event of one of the support frequencies being absent, support is taken from the sole support frequency to be detected and which may equally well be the support frequency that is above or the support frequency that is below the terminal under consideration. The emission frequency of the terminal under consideration then remains unchanged, at least in theory, so long as said terminal is emitting, even when both adjacent terminals have ceased to emit. As a result, at any instant, only some of the message channels of the network are occupied by transmitting a message, with some other channels being unoccupied at the same instant, except when all channels are busy. The spectrum range of the network must have room for these unoccupied channels. The range may be large if the number of terminals is large and if the message spectrum width is large.
That prior document mentions that when both adjacent terminals cease to emit, the frequency of the terminal under consideration may drift until it comes to be supported by the frequency of another terminal which is then emitting, or on a fixed base frequency. Slow and controlled drift of the emission frequencies could then systematically push a stack formed by said frequencies towards a predetermined end of the frequency range of the network.
However no means have been proposed that are useful for achieving such drift.
In said first known network, the emission frequency of the terminal under consideration is established after the frequency range of the network has been scanned. Before beginning a call the terminal itself performs said scanning. This enables it to identify possible emission frequencies that are unoccupied and are therefore available for emitting the message to be transmitted. An emission frequency identified in this way is selected and is initially used for sending calling signalling which includes at least the address of the called terminal. The time taken for scanning increases the access time of terminals to the network, i.e. the time which elapses on average between the instant at which a terminal receives an instruction from the associated user peripheral to transmit a message and the instant at which said transmission begins.
A second known network is described in a second prior document constituted by an article "Protocols for very high-speed optical fiber local area networks using a passive star topology" (Isam M. I. Habbab, Mohsen Kavehrad, and Carl-Erik E. Sundberg, Journal of Lightwave Technology, Vol. LT-5, No. 12, December 1987, pp. 1782-1793).
The spectrum range of said second known network includes a plurality of message channels together with a signalling channel and it appears that the positions of the channels are predetermined. The signalling channel is reserved for transmitting signalling, in particular for enabling a calling terminal to indicate the message channel that is to be used for a call to the called terminal.
The present invention has the following objects in particular:
facilitating implementation and operation of an optical fiber communications network with frequency multiplexing;
limiting the size of the frequency range of such a network; and
limiting the access time of a terminal to the network.